A person that is the fattest in a current area or a person that eats a shit ton out of everyone else. Easy to interpret by looking at a person literally bigger than everyone else or by who out-eats everybody.

Person 1: Oh shit that guys so fat
Person 2: What a Boombie!!
Or
Person 1: Dang why do you eat SO MUCH?
Person 2: You eat like a boombie